1. Field of the Invention
The present invention relates generally to the field of surface acoustic wave devices. More particularly, it relates to the precision adjustment of the resonance frequency of surface acoustic wave devices.
2. Description of the Prior Art
Surface acoustic wave delay line type filters may be fabricated to be operable at frequencies up to 2500 megahertz and resonator type filters have been fabricated that are operable to 1500 megahertz with Q values approaching the material limits. For SAW filters, to be useful as systems components, it is essential to be able to precisely set the resonant and synchronous frequencies of these devices. Although careful manufacturing methods will enable these resonant frequencies to be set within 0.01% of the frequency desired, for many applications this is not sufficiently accurate.
Conventional trimming techniques suffer from various shortcomings. In one method known in the art, a rectangular gold film is placed in the cavity of a resonator, and then selectively etched, thereby increasing the velocity of the surface acoustic waves and hence the resonant frequency of the device. This method is complex, and due to the large cavity area required, only useful for low frequency devices.
In a second technique, high energy ions are implanted in the surface of the SAW device, causing amorphisation of the quartz surface with a reduction in mass loading due to swelling. Ion implantation, however, results in increased device aging rates; furthermore, ion implantation equipment is expensive and generally not readily available.
The present invention, directed to methods for frequency trimming SAW devices, provides simple processes that do not result in undue device aging, are useful for devices of any frequency, and do not require equipment that is expensive or not readily available.